compiler warnings (was: Re: [rfc] a few kern.mk and bsd.sys.mk related changes)

[Dieter BSD]

> maybe we find some nice -Wwarning options which are reasonable
> to have

-Wmissing-declarations
-Wimplicit

FreeBSD's gcc doesn't seem to have  -Wcoercion  ???
Bugzilla indicates that it was added years ago (2006?).

It would be really really nice if -static worked on (nearly) everything.

> and - most importantly - don't break tinderbox

That's a matter of fixing the bugs before adding the warning flags
to tinderbox.

Ports need attention.  The warnings I get there are frightening.